This invention relates to a lighting device for such discharge lamps as automotive head lamps.
The discharge lamp lighting device of the kind referred to has been disclosed in, for example, Japanese Patent Laid-Open Publication No. 8-298190 (corresponding to U.S. Pat. No. 6,066,921), in which the device comprises a lighting circuit which controls a source power from a DC power source to a required power of a voltage for the discharge lamp and converts it into an AC stabilized, a starting circuit for generating a required high voltage (for example, 20 kV) for starting the discharge lamp, and a lamp fixture for supporting the discharge lamp.
When the discharge lamp lighting device thus constituted is caused to operate in a state where the discharge lamp is not connected as yet, there occurs a phenomenon of discharging between electrodes to which the discharge lamp is to be connected, due to the high voltage (such as 20 kV) generated at the starting circuit.
Another conventional example which prevents the above phenomenon is disclosed in Japanese Patent Laid-Open Publication No. 8-41738, in which a lamp socket 1 incorporates in a casing behind a socket section 1a a starting circuit 2 constituted by a pulse transformer 2a and an igniter main circuit 2b for generating a pulse voltage at a primary winding of the pulse transformer 2a, a lighting circuit 5 comprising an inverter circuit for converting a source power of a DC power source 9 into an AC power is connected at one output line 4a to an outer electrode 6 in the socket section 1a and at the other output line 4b through a secondary winding 2P to a central electrode 7 in the center of the socket section 1a. The igniter main circuit 2b is constituted by connecting one of a pair of source side terminals for receiving a supplied power to a junction point between the lighting circuit 5 and the secondary winding 2P of the pulse transformer 2a, and the other to an auxiliary electrode 8 in the socket section 1a. This known device is so constituted that, when the discharge lamp (not shown) is mounted to the socket section 1a, the auxiliary electrode 8 is connected through a conducting part of the discharge lamp to the outer electrode 6 to supply the power from the lighting circuit 5 to the igniter main circuit 2b to cause the starting circuit 2 to operate, whereas, when the discharge lamp is not mounted as yet, the power is not supplied to the igniter main circuit 2b, and the starting circuit 2 will not be actuated.
When in this case, as disclosed in Japanese Patent No. 2,909,867, a high voltage circuit constituted by a starting winding of the transformer, rectifying circuit, resistor and diode for supplying the power to the starting circuit is provided to a DC boosting circuit of the light circuit, it is possible to supply a high voltage suitable for a pulse generation to the starting circuit (starting pulse generating circuit), and the pulse generating transformer and the like in the starting circuit can be dimensionally minimized.
While the foregoing discharge lamp lighting device according to Japanese Patent Laid-Open Publication No. 8-41738 can restrain the starting circuit from being actuated in the state where the discharge lamp is not mounted and is safer than the discharge lamp lighting device according to Japanese Patent Laid-Open Publication No. 8-298190, the arrangement remains in that the power supply to the igniter main circuit 2b of the starting circuit 2 is made by a direct application of the output voltage of the lighting circuit 5, so that the voltage applied to the igniter main circuit 2b of the starting circuit 2 cannot be optionally selected (in such lighting circuit as shown in FIG. 9 of Japanese Patent Laid-Open Publication No. 6-349586, for example, a voltage two times as high as the voltage supplied from the lighting circuit is applied to the primary side of the pulse transformer), the pulse transformer 2a for generating the high voltage pulse is required to be one large in the turn ratio, and as a result the pulse transformer 2a is enlarged in the dimensions. Otherwise, it is required to provide in the starting circuit 2 a boosting circuit, consequent to which the number of electronic parts increases to be enough for occupying a larger housing space.
Further, it has been unable to realize a safe lighting device only by providing the boosting circuit for the supply to the starting circuit in the lighting circuit in attempt to minimize the dimensions of the starting circuit as disclosed in Japanese Patent No. 2,909,867 since, in the event when the discharge lamp is not mounted, a high voltage of several ten kV (such as about 20 kV) is applied to the output end of the socket when the source power is supplied to the lighting device.
In the event where the pulse transformer which is heavy is incorporated in the lamp socket of the type incorporating the starting circuit or the lamp socket is dimensionally enlarged, there arise risks that, as the automotive head lamp, the optical axis of the discharge lamp is apt to be deviated due to vibration during the running, so as to be a cause of hindering the optical axis from being stable as shown in Japanese Patent Laid-Open Publication No. 8-315630, or that the high pulse voltage is applied to the output end of the socket when the discharge lamp is not mounted so as to be in lack of the safety as shown in Japanese Patent No. 2,909,867.
It is an object of the present invention to provide a discharge lamp lighting device minimized in dimensions and weight of the socket, decreased in the costs and made highly safe, by overcoming the foregoing problems.
According to the present invention, the above object is established by the device characterized in comprising a discharge lamp, a lighting circuit for lighting the discharge lamp, and a socket for mounting the discharge lamp and including a starting circuit for causing the discharge lamp to be lighted, wherein the lighting circuit has a pair of output lines for use with lighting outputs for stably lighting the discharge lamp and connected at least at one through the starting circuit to the discharge lamp, and a power supply means arranged for supplying the power to the starting circuit when the discharge lamp is mounted to the socket is additionally provided.
Other objects and advantages of the present invention shall become clear from the following description with reference to the embodiments shown in accompanying drawings.